Cylinder blocks are manufactured by casting. In the case of a water-cooled engine, a water jacket needs to be provided in a cylinder block. The water jacket is made upon cylinder block casting simultaneously by means of a core.
A water jacket core is radially positioned by means of a bore pin and secured, via a mounting flange being integral with the bore pin, to a casting mold by means of a bolt (see FIG. 1 of Patent Literature 1, for example).
FIG. 7 hereof is a cross-sectional view illustrating a conventional water jacket core. A cavity 109 is defined by casting molds 97, 98, 99, 100 for forming a cylinder block. A cylinder liner 101 is placed against an outer periphery of a bore pin BP. A water jacket core 102 is positioned in such a manner as to surround the cylinder liner 101. The water jacket core 102 is secured to the casting mold 100 by means of a bolt 103.
The water jacket core 102 includes a base part 104 and a core body part 105.
Referring now to FIG. 8, description will be made as to the configuration of base part 104 of the water jacket core 102. A plurality of holes 107 (four in this example) that corresponds to the bore pins BP is formed in a bottom surface of the base part 104. A plurality of female thread parts 108 is formed in such a manner as to surround the holes 107. Top ends of the bolts 103 shown in FIG. 7 are threaded into the female thread parts 108 for fixing the core 102 to the casting mold 100.
During cast-forming, molten aluminum is poured into the cavity 109. When the molten metal is hardened, casting 111 of a multi-cylinder engine cylinder block is provided.
Upon separation of the water jacket core 102 by pushing the casting 111 out, the core body part 105 of the water jacket core 102 is liable to tilt or incline outwardly, as indicated by the angle α relative to a cylinder axial line 112, as shown in FIG. 9. Thus, the water jacket core 102 encounters tensioning and a warp stress during push-separation of product casting, leading to the deterioration of mass production durability.
The present inventors have studied the causes of the undesired deformation and noted thermal flow. In FIG. 7, part of the heat of the molten metal is transmitted from the core body part 105 to the base part 104 and further to the casting mold 100. Then, the heart is transmitted from the base part 104 to the casting mold 100 as shown by arrows in FIG. 10.
On the other hand, a Y-shaped part 113 provided between adjacent bores 107, 107 is positioned distantly from casting mold 100 and heated by the molten metal from plural directions. As a result, heat is accumulated in the Y-shaped part 113. It has been found that such heat causes the distortion as shown in FIG. 9.
Prevention of tilting of the water jacket core enables prevention of stress concentration in the core during the product casing push-out operation, thereby improving mass production durability.